Conservation Efforts May Have Backfired for Spanish Toad

The Mallorcan midwife toad caused quite a stir when it was discovered alive 30 years ago. Previously known only through fossils, the toad--whose males carry the eggs instead of females--was considered long extinct. The discovery sparked an all-out search across the Spanish island of Mallorca for this living fossil and, subsequently, prompted ongoing conservation and monitoring programs to preserve the 500 breeding pairs that currently exist. The Durrell Wildlife Conservation Trust in Jersey, U.K., established a successful breeding program, reintroducing animals to Mallorca to boost their numbers in the early 1990s. The toad's revival became one of conservation’s great success stories.

But now it appears that all has not gone according to plan. The first clue that something was amiss came in 2004, when fungus expert Matthew Fisher of Imperial College London discovered a chytrid fungus in a dead Mallorcan midwife toad from the island. Chytrid is widely blamed for the decline of amphibians worldwide. When Fisher and his colleagues screened Mallorcan midwife toads across the island, they found that the disease existed in epidemic proportions in two limestone gorges, was rare in two more gorges, and was nonexistent at about 17 more sites. This distribution suggested that the fungus had not had a chance to spread, indicating that it had been recently introduced to the island. The similar genetic makeup of the fungal specimens indicated a single source of infection, Fisher and his colleagues report today in Current Biology.

Fisher wondered whether reintroduced captive-bred toads could be to blame. He examined several toads that had died during the captive breeding programs and been preserved by the trust's researchers. Fisher and his colleagues found fungal DNA in three of the dead Mallorcan midwife toads and also in a preserved South African frog. When alive, several of these endangered frogs were kept in the same room as the midwife toad breeding colony, and they may have been the original source of the fungus. Shortly after they arrived in 1991, 23 Mallorcan midwife toads died, likely from fungal infection, says Fisher.

Yet no one at the time even knew the fungus existed, as it was only described as an emerging amphibian pathogen in 1998. Some captive midwife toads were released into one of the gorges where the fungus is now rampant, but the fungal DNA in the archived captive-bred toads was too old and degraded to determine if it matched that found on Mallorca.

Fortunately, the fungus has not yet proved as deadly to the Mallorcan toads as it has to amphibians elsewhere in the world. But the discovery of its spread is a sober reminder that conservationists eager to save amphibians must take extra care. "This study is the first documentation of an international reintroduction for conservation purposes that has resulted in the transmission of chytrid," says Robin Moore, amphibian conservation officer for Conservation International in Arlington, Virginia. Adds Kevin Zippel, program director of an international captive breeding program called Amphibian Ark: "It's a much-needed wake-up call. We must do everything in our power to assess and minimize the risks."